Picosecond fluorescence kinetics in spinach chloroplasts at room temperature. Effects of phosphorylation

The fast and slow kinetics of chlorophyll a fluorescence induction in plants, algae and cyanobacteria: a viewpoint

The light-induced/dark-reversible changes in the chlorophyll (Chl) a fluorescence of photosynthetic cells and membranes in the mus-to-several min time window (fluorescence induction, FI; or Kautsky transient) reflect quantum yield changes (quenching/de-quenching) as well as changes in the number of Chls a in photosystem II (PS II; state transitions). Both relate to excitation trapping in PS II and the ensuing photosynthetic electron transport (PSET), and to secondary PSET effects, such as ion translocation across thylakoid membranes and filling or depletion of post-PS II and post-PS I pools of metabolites. In addition, high actinic light doses may depress Chl a fluorescence irreversibly (photoinhibitory lowering; q(I)). FI has been studied quite extensively in plants an algae (less so in cyanobacteria) as it affords a low resolution panoramic view of the photosynthesis process. Total FI comprises two transients, a fast initial (OPS; for Origin, Peak, Steady state) and a second slower transient (SMT; for Steady state, Maximum, Terminal state), whose details are characteristically different in eukaryotic (plants and algae) and prokaryotic (cyanobacteria) oxygenic photosynthetic organisms. In the former, maximal fluorescence output occurs at peak P, with peak M lying much lower or being absent, in which case the PSMT phases are replaced by a monotonous PT fluorescence decay. In contrast, in phycobilisome (PBS)-containing cyanobacteria maximal fluorescence occurs at M which lies much higher than peak P. It will be argued that this difference is caused by a fluorescence lowering trend (state 1 --> 2 transition) that dominates the FI pattern of plants and algae, and correspondingly by a fluorescence increasing trend (state 2 --> 1 transition) that dominates the FI of PBS-containing cyanobacteria. Characteristically, however, the FI pattern of the PBS-minus cyanobacterium Acaryochloris marina resembles the FI patterns of algae and plants and not of the PBS-containing cyanobacteria.

Heat stress induces in leaves an increase of the minimum level of chlorophyll fluorescence, Fo: A time-resolved analysis

A time-resolved study of the effects of heat stress (23 to 50°C) on Fo level of chlorophyll fluorescence of leaves having different antenna content has been performed in order to elucidate the causes of heat induced increase of Fo in vivo. The multi-exponential deconvolution of the decays after a picosecond flash at Fo have shown that the best fit in both wild-type and the mutant chlorina F2 of barley leaves is obtained with three components in the temperature range utilized (100, 400 and 1200 ps at 23°C). In intermittent light greened pea leaves, a fourth long lifetime component (4 ns at 23°C) is needed. The comparison of the three types of leaves at 23°C shows that the content of the LHCII b complex does not affect the lifetimes of the two main components (100 and 400 ps) and affects their preexponential factors. This result suggests that in the PS II unit the exciton transfer from LHC IIb to the rest of the antenna is irreversible. The effects of heat stress on individual lifetime components, Ti, included several changes. Utilizing for PS II unit an extended 'Reversible Radical Pair' model, having three compartments, to interpret the variations of Ti and Ai induced by temperature increases, it can be inferred that heat determines: (i) an irreversible disconnection of a monor antenna complex which is not the LHC IIb complex, this effect is induced by temperatures higher than 40°C; (ii) a decrease of the quantum efficiency of Photosystem II photochemistry which is due to several effects: a decrease of the rate of charge separation, an increase of P(+)I(-) recombination rate constant and a decrease of the stabilization of charges. These effects on Photosystem II photochemistry start to occur above 30°C and are partially reversible.

A demonstration of photosynthetic state transitions in nature : Shading by photosynthetic tissue causes conversion to state 1

Photosynthetic state transitions occurring in nature are demonstrated. Chenopodium album leaves converted to state 1 and Ailanthus altissima leaves converted to an intermediate position between state 2 and state 1 at a time of day when these leaves were shaded by the canopy on a sunny day, while both plants' leaves were in state 2 at a time of day when they were not shaded. Filtering of white light by flasks of green algae also converted the light from causing state 2 in Chlorella vulgaris to causing state 1. Thus, light absorption by photosynthetic tissue can convert the natural light environment to one that causes state 1 in green plants.However, light absorption by water, by itself, up to a depth of 4.3m, does not change the light 2 character of sunlight.

Organization of the photosynthetic apparatus of the chlorina-f2 mutant of barley using chlorophyll fluorescence decay kinetics

The time-resolved chlorophyll fluorescence emission of higher plant chloroplasts monitors the primary processes of photosynthesis and reflects photosynthetic membrane organization. In the present study we compare measurements of the chlorophyll fluorescence decay kinetics of the chlorophyll-b-less chlorina-f2 barley mutant and wild-type barley to investigate the effect of alterations in thylakoid membrane composition on chlorophyll fluorescence. Our analysis characterizes the fluorescence decay of chlorina-f2 barley chloroplasts by three exponential components with lifetimes of approx. 100 ps, 400 ps and 2 ns. The majority of the chlorophyll fluorescence originates in the two faster decay components. Although photo-induced and cation-induced effects on fluorescence yields are evident, the fluorescence lifetimes are independent of the state of the Photosystem-II reaction centers and the degree of grana stacking. Wild-type barley chloroplasts also exhibit three kinetic fluorescence components, but they are distinguished from those of the chlorina-f2 chloroplasts by a slow decay component which displays cation- and photo-induced yield and lifetime changes. A comparison is presented of the kinetic analysis of the chlorina-f2 barley fluorescence to the decay kinetics previously measured for intermittent-light-grown peas (Karukstis, K. and Sauer, K. (1983) Biochim. Biophys. Acta 725, 384-393). We propose that similarities in the fluorescence decay kinetics of both species are a consequence of analogous rearrangements of the thylakoid membrane organization due to the deficiencies present in the light-harvesting chlorophyll a/b complex.